The present invention is primarily directed to a twist drill aligning bushing, and, more particularly, the invention is directed to such a bushing having a reversible cylindrical guide member removably positioned in a holder for accurately guiding a high speed twist drill or other tool during material removal from a workpiece. The structure shown can also be used for other types of bushing applications, such as reamer bushings.
The use of a bearing or bushing for accurately guiding a twist drill or the like through a jig plate or similar support is disclosed, for example, in U.S. Pat. No. 2,737,425, such patent teaching a coil-form liner positioned within an opening of a jig plate. Another type of prior art drill bushing, i.e. a potted bushing, is disclosed in U.S. Pat. No. 3,322,001, such bushing actually being potted in a plastic mounting a jig plate. Still a further prior art drill bushing includes a guide cylinder having an integral collar portion formed thereon, the guide cylinder being insertable into a jig plate opening and the collar being used to secure the bushing to the jig plate. Drill bushings of the latter type are relatively expensive because they are usually machined from expensive tool steel, such machining including turning the outside of the bushing, drilling the inside thereof, and otherwise forming the knurled collar, chamfers, undercuts and radii, therein. Moreover, it is usually desirable to heat treat a drill bushing for increased strength, but due to the larger thickness and diameter of the material at the collar relative to the thickness and diameter of the material at the guide cylinder, non-uniform heat treating results usually are obtained, whereby the drill bushing itself may become somewhat distorted and may even crack at the intersection of the two differently sized parts.
When a drill bushing is used to guide a high speed rotating twist through a jig plate to a workpiece, the wobble of the twist drill during entrance into the bushing and/or during operation will cause the bushing to wear at the end removed from the workpiece, whereupon the inner diameter of the cylindrical guide at the removed end no longer will be a true cylinder but rather will take on a conical or funnel-like shape, which reduces the accurate alignment function of the drill bushing. When the prior art drill bushings become too worn in this manner, they are usually discarded and replaced in the jig plate by another new, relatively expensive drill bushing.
Moreover, in order to stock an adequate supply of differently sized prior art drill bushings to meet the requirements of its customers, a supplier may have to tie up a large amount of money and space because drill bushings have different inner and outer diameters and different depth or length dimensions, depending on the dimensional parameters of the jig plate openings, of the twist drills, and of the position of the jig vis-a-vis the workpiece.